Why don't guys I like, like me?

Most of my life I have had a really hard time dating. Guys I've liked have never liked me back. A lot of people tell me what a great personality I have and how I have so much going for me. My older co-workers tell me how great I am and how dating should be easy for me. It's nice to hear but it makes me wonder why can't I get a guy to like me if I'm so great :( I think I'm cute even though I'm overweight and I feel confident when I'm not trying to impress anyone. I feel good about myself till I try to date someone. I've had a couple crushes at my current work place and it seems like when I try to get to know them they start pulling away. I hate that every time I think someone is flirting with me they are just being nice to me. Why can't I tell the difference. What am I doing wrong?